It is explained a little in this post, there are only certain types that are built in to work with the viewer.  The answer is that it isn’t configurable and office documents are not in the list to work through the viewer.

Hi, I attached serveral types of files under phurchase order :

pdf, doc, txt

and find only *.pdf] file will show the “view” button.

Text files (*.txt) should also work I think. As well as HTML files, if I remember correctly.